1. Field of the Invention
The present invention relates to a vertical magnetic recording medium suitable for a magnetic disk.
2. Description of the Related Art
In recent years, a much larger capacity and a much smaller size have been required for had disk drives, as information devices such as personal computers and work stations have progressed. In magnetic disks, much higher density is also required.
However, in a longitudinal magnetic recording method which has been being widely used, problems arise in that miniaturization of recording bits incurs heat fluctuation in recording magnetization and that a higher coercive force which may exceed the recording ability of a recording head is required, when realizing a high recording density. Hence, a vertical magnetic recording method has been discussed as a magnetic recording method capable of greatly improving the recording density while solving the problems described above. As a kind of vertical magnetic recording medium which realizes this method, there is a vertical magnetic recording medium having a two-layer structure which is composed of a soft magnetic film with a high magnetic permeability and a vertical magnetization film with a high vertical anisotropy.
FIG. 1 is a schematic cross sectional view showing an example of a conventional vertical magnetic recording medium. The vertical magnetic recording medium 21 is composed of a lower soft magnetic film 23 and a vertical magnetization film 24 which are laminated in an order on a substrate 22 made of non-magnetic material. For example, a NiFe film is used as the lower soft magnetic film 23, and CoCr-based alloy is used for the vertical magnetization film 24 (Nippon-Ouyou-Jiki-Gakkai-Shi, Vol. 8, No. 1, 1984, pp. 17-22).
This vertical magnetic recording medium 21 achieves recording more easily than a conventional longitudinal magnetic recording method because of existence of the lower soft magnetic film 23. That is, this vertical magnetic recording medium 21 can easily perform recording since it has much greater vertical magnetic anisotropic energy than the magnetic anisotropic energy of a conventional longitudinal magnetic recording medium in the longitudinal direction and also has much greater coercive force in the direction vertical to the film surface than a conventional longitudinal magnetic recording medium. Therefore, the vertical magnetic recording medium 21 can be stronger against heat fluctuation than a medium according to the longitudinal magnetic recording method. To deal with the problem of the heat fluctuation, it tends to use a film having high vertical magnetic anisotropic energy and coercive force in the direction vertical to the film surface greater than those of a CoCr-based film which has been the main trend of vertical magnetic films.
Meanwhile, in the conventional vertical magnetic recording medium using a vertical magnetization film which has great vertical magnetic anisotropic energy and large coercive force in the direction vertical to the film surface, the magnetic permeability xcexc and film thickness xcex4b of the lower soft magnetic film must be set to greater values in order to maintain a recording sensitivity, than in the case of using a conventional vertical magnetization film.
Where a practical medium manufacturing process is considered, the lower soft magnetic film should be thinner as much as possible. This is because a manufacturing process of the vertical magnetization film formed on the lower soft magnetic film becomes easier. The vertical magnetization film determines the recording/reproducing ability at a high recording density, when the lower soft magnetic film is thinned.
In conjunction with the above description, a vertical magnetic recording medium is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 4-283413). In this reference, a lower soft magnetic lining layer (12) and a vertical magnetization film (13) are formed in order on a non-magnetic material substrate (11). After a magnetic polar surface layer of the lower soft magnetic layer (12) is removed by an ion etching method or an inverse sputtering method, the vertical magnetization film (13) is formed thereon. The coercive force of the vertical magnetization film (13) becomes gradually smaller into a direction of the phase boundary with the lower soft magnetic layer (12) from the surface of the vertical magnetization film (13). Also, the saturation magnetization becomes gradually larger into a direction of the phase boundary with the lower soft magnetic layer (12) from the surface of the vertical magnetization film (13).
Also, a vertical magnetic recording medium is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 6-139542). In this reference, a lower soft magnetic layer (12) and a vertical magnetization film (13) are formed in order on a non-magnetic material substrate (11). The lower soft magnetic layer (12) has the relative permeability in a range of 20 to 1000 and the saturation magnetic flux density of 10 kG or more.
Also, a vertical magnetic recording medium is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 11-149628). In this reference, the vertical magnetic recording medium (20) is composed of a substrate (22), a lower soft magnetic film (24) formed on the substrate (22), and a vertical magnetization film 28 formed on the lower soft magnetic film (24). The lower soft magnetic film (24) is provided not to have a non-magnetic wall structure. The coercive force of the lower soft magnetic film (24) is equal to or less than 300 Oe.
An object of the present invention is to provide a vertical magnetic recording medium which can comply with thinning of the lower soft magnetic film.
Another object of the present invention is to provide a vertical magnetic recording medium which has large vertical magnetic anisotropic energy and large coercive force in the direction vertical to the film surface.
Still another object of the present invention is to provide a vertical magnetic recording medium which can eliminate the problem of heat fluctuation.
In order to achieve an aspect of the present invention, a vertical magnetic recording medium includes a soft magnetic film formed on a substrate, and a vertical magnetization film formed on the soft magnetic film, xcexcxc2x7xcex4bxe2x89xa71000, when xcexc is a permeability of the soft magnetic film, and xcex4b [nm] is a film thickness of the soft magnetic film.
Here, it is desirable that the permeability xcexc of the soft magnetic film is 5xe2x89xa6xcexcxe2x89xa6200. Also, it is desirable that the film thickness xcex4b of the soft magnetic film is equal to or less than 500 nm.
Also, it is desirable that vertical magnetic anisotropy energy Ku [erg/cc] of the vertical magnetization film is 1xc3x97107xe2x89xa6Kuxe2x89xa67xc3x97108. Also, it is desirable that coercive force Hc [kOe] of the vertical magnetization film in the vertical direction to a surface of the vertical magnetization film is 5xe2x89xa6Hcxe2x89xa610.
Also, the vertical magnetization film may include FePt alloy. In this case, it is desirable that the vertical magnetization film comprises x at %Fe-(100-x) at %Pt alloy, where 40xe2x89xa6xxe2x89xa660. It is more desirable that the vertical magnetization film comprises 50at %Fe-50at %Pt alloy, where x=50 at %.
Also, the vertical magnetization film may include RCo alloy, where R is one or more selected from the group consisting of Y, Ce, Sm, La and Pr. In this case, the vertical magnetization film may include RCo5 alloy, where R is one or more selected from the group consisting of Y, Ce and Sm. Alternatively, the vertical magnetization film may include R2Co17 alloy, where R is one or more selected from the group consisting of Y, Ce, Sm, La and Pr.
Also, the soft magnetic film may include FeSiAl alloy. In this case, it is desirable that the vertical magnetization film comprises 84.9wt %Fe-xwt %Si-(15.1-x)wt %Al alloy, where 8.0xe2x89xa6xxe2x89xa612.0. It is more desirable that the vertical magnetization film may include 84.9wt %Fe-9.6wt %Si-5.5wt %Al.
Also, the soft magnetic film may include CoNiFe alloy. Especially, it is desirable that the soft magnetic film may include 62at %Co-12at %Ni-26at %Fe alloy.
In order to achieve another aspect of the present invention, a vertical magnetic recording medium includes a soft magnetic film formed on a substrate, and a vertical magnetization film formed on the soft magnetic film, and xcexcxc2x7xcex4bxe2x89xa71000 when xcexc is a permeability of the soft magnetic film, and xcex4b [nm] is a film thickness of the soft magnetic film. In addition, the permeability xcexc of the soft magnetic film is 5xe2x89xa6xcexcxe2x89xa6200, and the film thickness xcex4b of the soft magnetic film is equal to or less than 500 nm.
Here, it is desirable that the vertical magnetization film may include FePt alloy or RCo alloy, where R is one or more selected from the group consisting of Y, Ce, Sm, La and Pr.
Also, it is desirable that the soft magnetic film may include FeSiAl alloy or CoNiFe alloy.
Also, it is desirable that vertical magnetic anisotropy energy Ku [erg/cc] of the vertical magnetization film is 1xc3x97107xe2x89xa6Kuxe2x89xa67xc3x97108, and coercive force Hc [kOe] of the vertical magnetization film in the vertical direction to a surface of the vertical magnetization film is 5xe2x89xa6Hcxe2x89xa610.